This invention generally relates to devices for vaporizing liquid cryogens, and is particularly concerned with a compact device for vaporizing a liquid cryogen and superheating the resulting vapor to a temperature useful for defrosting and/or heating the evaporator coil of a mobile cryogenic refrigeration system.
Devices for vaporizing a liquid cryogen and superheating the resulting vapor are known in the prior art. Such devices may be used in cryogenic refrigeration systems where a cryogenic liquid is expanded in an evaporator coil to cool a conditioned space, which may be a truck or tractor-trailer. In these systems, unwanted ice deposits accumulate on the exterior surface of the evaporator coil from ambient humidity. A cryogenic vaporizer and superheater is used to create hot gases that circulate through the evaporator coil to melt the unwanted ice deposits. Such devices comprise a vaporizer coil formed from a plurality of windings of metal tubing which are heated by ambient air to vaporize the liquid cryogen into a gas. The outlet of the vaporizer coil is connected to the inlet of a superheating coil likewise formed by windings of metal tubing. The windings of the vaporizer and superheating coils are convectively heated by the combustion gases of a combustion-type heat source, such as a propane burner. The inlet of the vaporizer coil is connected via a mode valve to the source of the liquid cryogen used in the refrigeration system, which may be liquid carbon dioxide, liquid nitrogen, or the like. The outlet of the superheating coil may in turn be connected to the evaporator coil of the cryogenic refrigeration system to either defrost the evaporator coil, or to heat it when it is desired to heat the conditioned space.
In operation, when the defrost or heating cycle of the refrigeration system is actuated, liquid cryogen is admitted to the vaporizer coil while ambient air circulates around the windings of the coil. The resulting convective transfer of heat causes the liquid cryogen entering the vaporizer coil to boil and to vaporize so that it leaves the outlet conduit of this coil in a gaseous state. The gaseous cryogen then flows to the superheating coil where more convective heat transfer is accomplished by the contact of the hot combustion gases of the propane burner on the exterior surface of the superheating coil. By the time the gaseous cryogen exits the superheating coil, it is at a temperature sufficiently high to supply the desired quantity of heat to defrost or heat the evaporator coil of the refrigeration system. Various flow control valves in the system isolate the evaporator coil from liquid cryogen while directing the superheated gas into the coil. After a sufficient amount of superheated gas has been circulated through the interior of the evaporator coil to melt the unwanted ice deposits on its exterior, or to heat the conditioned space to a desired temperature, the positions of the valves in the system are changed and the propane burner is shut off until the next defrosting or heating cycle.
While such prior art vaporizing and superheating devices have generally shown themselves to be effective for their intended purpose, the applicants have observed two major shortcomings in the design of these devices which significantly compromises their effectiveness.
First, under certain temperature and humidity conditions, a layer of moisture or ice formed from ambient humidity can build up on the surface of the vaporizer coil. The resulting moisture layer can seriously interfere with the transfer of further ambient heat to the liquid cryogen causing liquid cryogen to enter the superheating coil. Intuitively, it would appear that the exposure of the windings of the superheating coil to the flames of the propane burner would effectively vaporize and superheat any liquid cryogen that spilled over from the vaporizer coil. However, because water is a byproduct of the combustion of most fossil fuels, when such water-containing combustion gases come into contact with a coil containing a cryogenic liquid, the temperature of the cryogenic liquid is low enough (e.g., -65.degree. F. in the case of liquid carbon dioxide) to cause the formation of a layer of ice and water on the surface of the coil. The presence of this dynamic film of water and ice seriously impairs the efficiency of the exchange of heat between the combustion gases and the surface of the metal tubing forming the vaporizer coil. Over a period of time the problem gradually worsens and results in liquid CO.sub.2 carry over from the vaporizer to the superheater and eventually the evaporator coil, rendering the entire heating/defrost operation ineffective.
Secondly, the applicants have observed that the configuration of the vaporizer and superheating coils in such prior art devices requires a considerable amount of volume. This is a particularly disadvantageous feature in the context of a cryogenic refrigeration system used on a truck or other vehicle, where a high degree of compactness is desirable to maximize the cost efficiency of the system.
Clearly, there is a need for an improved device for vaporizing and superheating a cryogenic liquid which realizes a high degree of heat transfer efficiency. Moreover, it would be desirable if such a device could be compactly constructed and installed in the limited space available in a mobile cryogenic refrigeration system so as to maximize the cost efficiency of the system.